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https://github.com/golang/go
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77595e462b
Before this CL, if the system resolver does a very slow DNS lookup for a particular host, all subsequent requests for that host will hang waiting for that lookup to complete. That is more or less expected when Dial is called with no deadline. When Dial has a deadline, though, we can accumulate a large number of goroutines waiting for that slow DNS lookup. Try to avoid this problem by restarting the DNS lookup when it is redone after a deadline is passed. This CL also avoids creating an extra goroutine purely to handle the deadline. No test because we would have to simulate a slow DNS lookup followed by a fast DNS lookup. Fixes #8602. LGTM=bradfitz R=bradfitz, mikioh.mikioh CC=golang-codereviews, r, rsc https://golang.org/cl/154610044
110 lines
2.8 KiB
Go
110 lines
2.8 KiB
Go
// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package net
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import "sync"
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// call is an in-flight or completed singleflight.Do call
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type call struct {
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wg sync.WaitGroup
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// These fields are written once before the WaitGroup is done
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// and are only read after the WaitGroup is done.
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val interface{}
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err error
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// These fields are read and written with the singleflight
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// mutex held before the WaitGroup is done, and are read but
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// not written after the WaitGroup is done.
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dups int
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chans []chan<- singleflightResult
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}
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// singleflight represents a class of work and forms a namespace in
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// which units of work can be executed with duplicate suppression.
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type singleflight struct {
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mu sync.Mutex // protects m
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m map[string]*call // lazily initialized
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}
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// singleflightResult holds the results of Do, so they can be passed
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// on a channel.
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type singleflightResult struct {
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v interface{}
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err error
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shared bool
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}
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// Do executes and returns the results of the given function, making
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// sure that only one execution is in-flight for a given key at a
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// time. If a duplicate comes in, the duplicate caller waits for the
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// original to complete and receives the same results.
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// The return value shared indicates whether v was given to multiple callers.
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func (g *singleflight) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
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g.mu.Lock()
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if g.m == nil {
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g.m = make(map[string]*call)
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}
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if c, ok := g.m[key]; ok {
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c.dups++
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g.mu.Unlock()
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c.wg.Wait()
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return c.val, c.err, true
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}
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c := new(call)
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c.wg.Add(1)
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g.m[key] = c
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g.mu.Unlock()
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g.doCall(c, key, fn)
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return c.val, c.err, c.dups > 0
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}
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// DoChan is like Do but returns a channel that will receive the
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// results when they are ready.
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func (g *singleflight) DoChan(key string, fn func() (interface{}, error)) <-chan singleflightResult {
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ch := make(chan singleflightResult, 1)
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g.mu.Lock()
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if g.m == nil {
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g.m = make(map[string]*call)
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}
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if c, ok := g.m[key]; ok {
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c.dups++
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c.chans = append(c.chans, ch)
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g.mu.Unlock()
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return ch
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}
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c := &call{chans: []chan<- singleflightResult{ch}}
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c.wg.Add(1)
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g.m[key] = c
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g.mu.Unlock()
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go g.doCall(c, key, fn)
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return ch
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}
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// doCall handles the single call for a key.
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func (g *singleflight) doCall(c *call, key string, fn func() (interface{}, error)) {
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c.val, c.err = fn()
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c.wg.Done()
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g.mu.Lock()
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delete(g.m, key)
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for _, ch := range c.chans {
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ch <- singleflightResult{c.val, c.err, c.dups > 0}
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}
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g.mu.Unlock()
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}
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// Forget tells the singleflight to forget about a key. Future calls
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// to Do for this key will call the function rather than waiting for
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// an earlier call to complete.
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func (g *singleflight) Forget(key string) {
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g.mu.Lock()
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delete(g.m, key)
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g.mu.Unlock()
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}
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